U.S. patent application number 11/374416 was filed with the patent office on 2007-09-20 for system and method for moving a component through a setpoint profile, lithographic apparatus and device manufacturing method.
This patent application is currently assigned to ASML Netherlands B.V.. Invention is credited to Marcel Francois Heertjes, Yin-Tim Tso, Edwin Teunis Van Donkelaar.
Application Number | 20070219647 11/374416 |
Document ID | / |
Family ID | 38518939 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070219647 |
Kind Code |
A1 |
Heertjes; Marcel Francois ;
et al. |
September 20, 2007 |
System and method for moving a component through a setpoint
profile, lithographic apparatus and device manufacturing method
Abstract
A system to move a component in accordance with a setpoint
profile including a plurality of target states of the component,
each of the plurality of target states to be substantially attained
at one of a corresponding sequence of target times, is presented.
The system includes a displacement device to move the component
according to the setpoint profile; a storage device containing a
library of feedforward data; a signal generating part configured to
identify a plurality of time segments of the setpoint profile that
correspond to entries in the library of feedforward data, and
access the entries in order to construct a feedforward signal; and
a feedforward control system to control the operation of the
displacement device by reference to the feedforward signal
constructed by the signal generating part.
Inventors: |
Heertjes; Marcel Francois;
(Best, NL) ; Tso; Yin-Tim; (Eindhoven, NL)
; Van Donkelaar; Edwin Teunis; (Den Haag, NL) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML Netherlands B.V.
Veldhoven
NL
|
Family ID: |
38518939 |
Appl. No.: |
11/374416 |
Filed: |
March 14, 2006 |
Current U.S.
Class: |
700/44 |
Current CPC
Class: |
G05B 19/371 20130101;
G05B 2219/41451 20130101 |
Class at
Publication: |
700/044 |
International
Class: |
G05B 13/02 20060101
G05B013/02 |
Claims
1. A system configured to move a component in accordance with a
setpoint profile that includes a plurality of target states of said
component, each of said plurality of target states to be
substantially attained at one of a corresponding sequence of target
times, said system comprising: a displacement device configured to
move said component according to said setpoint profile; a data
storage device that includes a library of feedforward data; a
signal generator configured to (a) identify a plurality of time
segments of said setpoint profile that correspond to entries in
said library of feedforward data, and (b) access said entries in
order to construct a feedforward signal; and a feedforward control
system configured to control said displacement device based on said
feedforward signal that is constructed by said signal
generator.
2. A system according to claim 1, wherein said plurality of target
states include at least one of target positions of said component,
target velocities of said component, and target accelerations of
said component.
3. A system according to claim 1, wherein said signal generator
comprises: a trigger detection system configured to detect segments
of said setpoint profile that correspond to entries in said library
of feedforward data; and an input device configured to insert the
entries into said feedforward signal when a corresponding segment
is detected by said trigger detection system.
4. A system according to claim 1, wherein said entries in said
library include feedforward data for at least one of the following
types of setpoint profile segment: a constant velocity profile, a
constant acceleration profile, a constant deceleration profile, a
jerk phase, a djerk phase, and profiles characterized by finite
values of higher order derivatives of position with respect to
time.
5. A system according to claim 1, wherein at least a subset of said
entries in said library are based on previous calibration
measurements.
6. A system according to claim 5, wherein said calibration
measurements comprise iteratively learnt data.
7. A lithographic projection apparatus arranged to project a
pattern from a patterning device onto a substrate, comprising: (a)
a movable support configured to hold said patterning device; and
(b) a system configured to move said movable support in accordance
with a setpoint profile that includes a plurality of target states
of said movable support, each of said plurality of target states to
be substantially attained at one of a corresponding sequence of
target times, said system comprising (i) a displacement device
configured to move said movable support according to said setpoint
profile; (ii) a data storage device that includes a library of
feedforward data; (iii) a signal generator configured to (1)
identify a plurality of time segments of said setpoint profile that
correspond to entries in said library of feedforward data, and (2)
access said entries in order to construct a feedforward signal; and
(iv) a feedforward control system configured to control said
displacement device based on said feedforward signal that is
constructed by said signal generator.
8. A lithographic projection apparatus arranged to project a
pattern from a patterning device onto a substrate, comprising: (a)
a movable support configured to hold said substrate; and (b) a
system configured to move said movable support in accordance with a
setpoint profile that includes a plurality of target states of said
movable support, each of said plurality of target states to be
substantially attained at one of a corresponding sequence of target
times, said system comprising (i) a displacement device configured
to move said movable support according to said setpoint profile;
(ii) a data storage device that includes a library of feedforward
data; (iii) a signal generator configured to (1) identify a
plurality of time segments of said setpoint profile that correspond
to entries in said library of feedforward data, and (2) access said
entries in order to construct a feedforward signal; and (iv) a
feedforward control system configured to control said displacement
device based on said feedforward signal that is constructed by said
signal generator.
9. A method of moving a component in accordance with a setpoint
profile that includes a plurality of target states of said
component, each of said plurality of target states to be
substantially attained at one of a corresponding sequence of target
times, said method comprising: comparing a plurality of time
segments of said setpoint profile with entries in a library of
feedforward data; identifying time segments for which feedforward
data exist in said library; retrieving the feedforward data for the
identified time segments; constructing at least part of a
feedforward signal using said retrieved feedforward data; and
controlling a movement of said component according to said setpoint
profile based on said feedforward signal.
10. A method according to claim 9, wherein at least a subset of
said entries in said library are based on previous calibration
measurements.
11. A method according to claim 10, wherein said calibration
measurements are obtained by: moving said component according to a
setpoint profile time segment using a feedforward signal; measuring
a state of said component during said movement and determining a
difference between said measured state and a corresponding target
state of said component defined by said setpoint profile; refining
said feedforward signal according to said difference; repeating
said moving, measuring and refining until said difference is below
a target threshold; and storing the refined feedforward signal in
said library of feedforward data in correspondence with the
associated setpoint profile time segment.
12. A device manufacturing method comprising: (a) projecting a
pattern from a patterning device onto a substrate; and (b) moving a
movable support that is configured to hold said patterning device
in accordance with a setpoint profile that includes a plurality of
target states of said movable support, each of said plurality of
target states to be substantially attained at one of a
corresponding sequence of target times, said moving comprising (i)
comparing a plurality of time segments of said setpoint profile
with entries in a library of feedforward data; (ii) identifying
time segments for which feedforward data exists in said library;
(iii) retrieving the feedforward data for the identified time
segments; (iv) constructing at least part of a feedforward signal
using said retrieved feedforward data; and (v) controlling a
movement of said movable support according to said setpoint profile
based on said feedforward signal.
13. A device manufacturing method comprising: (a) projecting a
pattern from a patterning device onto a substrate; and (b) moving a
movable support configured to hold said substrate in accordance
with a setpoint profile that includes a plurality of target states
of said movable support, each of said plurality of target states to
be substantially attained at one of a corresponding sequence of
target times, said moving comprising (i) comparing a plurality of
time segments of said setpoint profile with entries in a library of
feedforward data; (ii) identifying time segments for which
feedforward data exists in said library; (iii) retrieving the
feedforward data for the identified time segments; (iv)
constructing at least part of a feedforward signal using said
retrieved feedforward data; and (v) controlling the movement of
said movable support according to said setpoint profile based on
said feedforward signal.
Description
FIELD
[0001] The present invention relates to a system for moving a
component through a setpoint profile and a lithographic apparatus
including such a system. The invention further relates to a method
for moving a component through a setpoint profile and a device
manufacturing method including such a method.
BACKGROUND
[0002] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. including part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at once, and so-called scanners, in which each target portion is
irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti-parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
[0003] In order to ensure good performance of the lithographic
apparatus, it is desirable to control the precision with which
components to be moved during exposure, such as the reticle stage
(patterning device table) containing the patterns needed for
illumination and the substrate table containing the substrates to
be illuminated, can be displaced. Under feedback control, the
movement of components is controlled using standard PID-based
control systems. However, to obtain nano-scale position accuracy,
with settling times of the order of milliseconds or lower,
feedforward control may be necessary.
[0004] In addition to the commonly used acceleration-, jerk-, and
even snap-based feedforward control designs (i.e. designs based on
acceleration and higher order derivatives of position with respect
to time), the application of iterative learning control as a means
for obtaining short settling times has been suggested. This
approach has the benefit that only limited system knowledge is
required to implement the feedforward control with high accuracy.
The method is based on iteratively learning a feedforward signal or
"force" by reference to a measured error signal (defined as a
measured deviation of the state of a component being moved from a
setpoint profile defining an intended time evolution of the state)
of sufficient duration to allow a degree of convergence. When the
learned signal is applied to the system or process it effectively
counteracts the repetitive contributions to the error signal. The
learned signal, which may be stored in a table for example,
corresponds to the particular setpoint profile used for the
learning.
[0005] Even for a given type of scanning in a lithography
apparatus, the setpoint profile is likely to vary as die lengths
and/or exposure velocities are varied. Where many different
profiles are needed, it is desirable to learn and store many
different feedforward tables. The situation is not restricted to
feedforward data derived from iterative learning, but will occur
whenever feedforward data is dependent on the setpoint profile.
SUMMARY
[0006] It is desirable to provide an improved system for dealing
with setpoint profile dependent feedforward data.
[0007] According to an embodiment of the invention, there is
provided a system for moving a component through a setpoint profile
including a plurality of target states of the component, each to be
substantially attained at one of a corresponding sequence of target
times, the system including: a displacement device for moving the
component according to the setpoint profile; a storage device (or
data storage device) containing a library of feedforward data; a
signal generating part (or signal generator) configured to identify
a plurality of time segments of the setpoint profile that
correspond to entries in the library of feedforward data, and
access the entries in order to construct a feedforward signal; and
a feedforward control system for controlling the operation of the
displacement device by reference to the feedforward signal
constructed by the signal generating part.
[0008] According to a further embodiment of the invention, there is
provided a lithographic projection apparatus arranged to project a
pattern from a patterning device onto a substrate, including: a
movable support for the patterning device; and a system for moving
the movable support through a setpoint profile including a
plurality of target states of the movable support, each to be
substantially attained at one of a corresponding sequence of target
times, the system including: a displacement device for moving the
movable support according to the setpoint profile; a storage device
containing a library of feedforward data; a signal generating part
configured to identify a plurality of time segments of the setpoint
profile that correspond to entries in the library of feedforward
data, and access the entries in order to construct a feedforward
signal; and a feedforward control system for controlling the
operation of the displacement device by reference to the
feedforward signal constructed by the signal generating part.
[0009] According to a further embodiment of the invention, there is
provided a lithographic projection apparatus arranged to project a
pattern from a patterning device onto a substrate, including: a
movable support for the substrate; and a system for moving the
movable support through a setpoint profile including a plurality of
target states of the movable support, each to be substantially
attained at one of a corresponding sequence of target times, the
system including: a displacement device for moving the movable
support according to the setpoint profile; a storage device
containing a library of feedforward data; a signal generating part
configured to identify a plurality of time segments of the setpoint
profile that correspond to entries in the library of feedforward
data, and access the entries in order to construct a feedforward
signal; and a feedforward control system for controlling the
operation of the displacement device by reference to the
feedforward signal constructed by the signal generating part.
[0010] According to a further embodiment of the invention, there is
provided a method of moving a component through a setpoint profile
including a plurality of target states of the component, each to be
substantially attained at one of a corresponding sequence of target
times, the method including: comparing a plurality of time segments
of the setpoint profile with entries in a library of feedforward
data and identifying time segments for which feedforward data
exists in the library; retrieving feedforward data for time
segments thus identified and constructing at least part of a
feedforward signal using the retrieved feedforward data; using the
feedforward signal to control the movement of the component
according to the setpoint profile.
[0011] According to a further embodiment of the invention, there is
provided a device manufacturing method including projecting a
pattern from a patterning device onto a substrate, including:
providing a movable support for the patterning device; and moving
the movable support through a setpoint profile comprising a
plurality of target states of the movable support, each to be
substantially attained at one of a corresponding sequence of target
times, the method including: comparing a plurality of time segments
of the setpoint profile with entries in a library of feedforward
data and identifying time segments for which feedforward data
exists in the library; retrieving feedforward data for time
segments thus identified and constructing at least part of a
feedforward signal using the retrieved feedforward data; using the
feedforward signal to control the movement of the movable support
according to the setpoint profile.
[0012] According to a further embodiment of the invention, there is
provided a device manufacturing method including projecting a
pattern from a patterning device onto a substrate, including:
providing a movable support for the substrate; and moving the
movable support through a setpoint profile comprising a plurality
of target states of the movable support, each to be substantially
attained at one of a corresponding sequence of target times, the
method including: comparing a plurality of time segments of the
setpoint profile with entries in a library of feedforward data and
identifying time segments for which feedforward data exists in the
library; retrieving feedforward data for time segments thus
identified and constructing at least part of a feedforward signal
using the retrieved feedforward data; using the feedforward signal
to control the movement of the movable support according to the
setpoint profile.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0014] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0015] FIGS. 2a-c depict a setpoint profile defined in terms of an
acceleration profile, a velocity profile and a position
profile;
[0016] FIG. 3 discloses a lithography apparatus with a signal
generating part and feedforward control system according to an
embodiment of the invention;
[0017] FIG. 4 depicts a storage device, signal generating part and
feedforward control system according to an embodiment of the
invention;
[0018] FIG. 5 depicts an iterative learning control algorithm in
block diagram representation;
[0019] FIG. 6 depicts a setpoint profile and typical corresponding
error signal;
[0020] FIG. 7 depicts error signals resulting from a feedforward
control system using two learned feedforward tables; and
[0021] FIG. 8 depicts error signals resulting from a feedforward
control system using feedforward data segments corresponding to the
jerk/djerk phase and part of the corresponding constant velocity
regions.
DETAILED DESCRIPTION
[0022] FIG. 1 schematically depicts a lithographic apparatus
according to one embodiment of the invention. The apparatus
includes an illumination system (illuminator) IL configured to
condition a radiation beam B (e.g. UV radiation or EUV radiation);
a support structure (e.g. a mask table) MT constructed to support a
patterning device (e.g. a mask) MA and connected to a first
positioner PM configured to accurately position the patterning
device in accordance with certain parameters; a substrate table
(e.g. a wafer table) WT constructed to hold a substrate (e.g. a
resist-coated wafer) W and connected to a second positioner PW
configured to accurately position the substrate in accordance with
certain parameters; and a projection system (e.g. a refractive
projection lens system) PS configured to project a pattern imparted
to the radiation beam B by patterning device MA onto a target
portion C (e.g. including one or more dies) of the substrate W.
[0023] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
[0024] The support structure supports, i.e. bears the weight of,
the patterning device. It holds the patterning device in a manner
that depends on the orientation of the patterning device, the
design of the lithographic apparatus, and other conditions, such as
for example whether or not the patterning device is held in a
vacuum environment. The support structure can use mechanical,
vacuum, electrostatic or other clamping techniques to hold the
patterning device. The support structure may be a frame or a table,
for example, which may be fixed or movable as required. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device."
[0025] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
[0026] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam which is reflected by the mirror matrix.
[0027] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system".
[0028] As here depicted, the apparatus is of a transmissive type
(e.g. employing a transmissive mask). Alternatively, the apparatus
may be of a reflective type (e.g. employing a programmable mirror
array of a type as referred to above, or employing a reflective
mask).
[0029] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more mask tables).
In such "multiple stage" machines the additional tables may be used
in parallel, or preparatory steps may be carried out on one or more
tables while one or more other tables are being used for
exposure.
[0030] The lithographic apparatus may also be of a type wherein at
least a portion of the substrate may be covered by a liquid having
a relatively high refractive index, e.g. water, so as to fill a
space between the projection system and the substrate. An immersion
liquid may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the projection system.
Immersion techniques are well known in the art for increasing the
numerical aperture of projection systems. The term "immersion" as
used herein does not mean that a structure, such as a substrate,
must be submerged in liquid, but rather only means that liquid is
located between the projection system and the substrate during
exposure.
[0031] Referring to FIG. 1, the illuminator IL receives a radiation
beam from a radiation source SO. The source and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser. In such cases, the source is not considered to
form part of the lithographic apparatus and the radiation beam is
passed from the source SO to the illuminator IL with the aid of a
beam delivery system BD including, for example, suitable directing
mirrors and/or a beam expander. In other cases the source may be an
integral part of the lithographic apparatus, for example when the
source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
[0032] The illuminator IL may include an adjuster AD for adjusting
the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may include various
other components, such as an integrator IN and a condenser CO. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0033] The radiation beam B is incident on the patterning device
(e.g., mask MA), which is held on the support structure (e.g., mask
table MT), and is patterned by the patterning device. Having
traversed the mask MA, the radiation beam B passes through the
projection system PS, which focuses the beam onto a target portion
C of the substrate W. With the aid of the second positioner PW and
position sensor IF (e.g. an interferometric device, linear encoder
or capacitive sensor), the substrate table WT can be moved
accurately, e.g. so as to position different target portions C in
the path of the radiation beam B. Similarly, the first positioner
PM and another position sensor (which is not explicitly depicted in
FIG. 1) can be used to accurately position the mask MA with respect
to the path of the radiation beam B, e.g. after mechanical
retrieval from a mask library, or during a scan. In general,
movement of the mask table MT may be realized with the aid of a
long-stroke module (coarse positioning) and a short-stroke module
(fine positioning), which form part of the first positioner PM.
Similarly, movement of the substrate table WT may be realized using
a long-stroke module and a short-stroke module, which form part of
the second positioner PW. In the case of a stepper (as opposed to a
scanner) the mask table MT may be connected to a short-stroke
actuator only, or may be fixed. Mask MA and substrate W may be
aligned using mask alignment marks M1, M2 and substrate alignment
marks P1, P2. Although the substrate alignment marks as illustrated
occupy dedicated target portions, they may be located in spaces
between target portions (these are known as scribe-lane alignment
marks). Similarly, in situations in which more than one die is
provided on the mask MA, the mask alignment marks may be located
between the dies.
[0034] The depicted apparatus could be used in at least one of the
following modes:
[0035] 1. In step mode, the mask table MT and the substrate table
WT are kept essentially stationary, while an entire pattern
imparted to the radiation beam is projected onto a target portion C
at one time (i.e. a single static exposure). The substrate table WT
is then shifted in the X and/or Y direction so that a different
target portion C can be exposed. In step mode, the maximum size of
the exposure field limits the size of the target portion C imaged
in a single static exposure.
[0036] 2. In scan mode, the mask table MT and the substrate table
WT are scanned synchronously while a pattern imparted to the
radiation beam is projected onto a target portion C (i.e. a single
dynamic exposure). The velocity and direction of the substrate
table WT relative to the mask table MT may be determined by the
(de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
[0037] 3. In another mode, the mask table MT is kept essentially
stationary holding a programmable patterning device, and the
substrate table WT is moved or scanned while a pattern imparted to
the radiation beam is projected onto a target portion C. In this
mode, generally a pulsed radiation source is employed and the
programmable patterning device is updated as required after each
movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
[0038] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0039] FIG. 2 illustrates what is meant by a setpoint profile.
Three schematic graphs are shown representing (from top to bottom)
the acceleration (FIG. 2a), velocity (FIG. 2b) and position (FIG.
2c) of a component while it is being moved through a simplified
setpoint profile. The setpoint profile, in this example, includes
three distinct regimes: an acceleration phase 2, a constant
velocity phase 4, and a deceleration phase 6. In general, the
setpoint profile may be characterized by a sequence of target
states that the component is intended to reach at particular target
times.
[0040] As mentioned above, accurate control of components to be
moved may be achieved using a feedforward control system. The
feedforward signal in such systems may be based either on explicit
system knowledge (based on factors such as the mass of the
component to be moved) or on feedforward data derived from previous
measurements (for example, an iterative learning scheme may be
employed). Feedforward data of the latter type is often dependent
on the particular setpoint profile that is currently being used, a
different set being needed if the setpoint profile is changed.
Where many different setpoint profiles are envisaged, a large
amount of feedforward data may have to be stored and many prior
measurements may have to be carried out in order to derive (or
"learn") all of the desired feedforward data.
[0041] According to an embodiment of the invention, it is possible
to cope with a large number of setpoint profiles using only a
limited amount of feedforward data. This is achieved by using a
switched feedforward data strategy by which feedforward data for a
given setpoint profile is built up from segments of feedforward
data extracted from a library of feedforward data. This approach is
based on the realization that in many practical situations
different applied acceleration profiles (setpoint profiles) have a
number of features in common: for example, constant
acceleration/deceleration phases (for fixed velocities) or constant
jerk/djerk phase (a "jerk" phase refers to a period of constant
rate of increase of acceleration and a "djerk" phase corresponds to
a period of constant rate of decrease of acceleration). Due to the
fact that the setpoints (to achieve maximum throughput) often apply
maximum jerk and acceleration levels, the slope of the
acceleration/deceleration phases are more or less fixed for a given
hardware setup, hence so is the time needed to reach the
steady-state acceleration level. A user of the lithography
apparatus using this technology may need different velocities
according to the particular process steps being performed but
maximum acceleration and jerk may still be applied for a
significant range of velocities. For very low velocity processes,
where throughput may be less important, the acceleration and/or
jerk may be scaled down. Differences between setpoint profiles,
such as in the length of the constant velocity phase, can be
considered as different compositions of these generic features.
[0042] An embodiment is described below where only two blocks of
feedforward data are derived and subsequently stored: one
containing an acceleration phase and one containing a deceleration
phase of a setpoint profile. Using only these two blocks of
feedforward data, it is shown that a number of setpoint profiles
with smaller constant velocity lengths can efficiently be handled.
In another embodiment, we consider the jerk phase and part of the
constant velocity phase along with the djerk phase and an equal
part of the corresponding constant velocity phase. A wide range of
other setpoint profiles may be dealt with an analogous way.
[0043] FIG. 3 illustrates an embodiment of the invention as applied
to a lithography apparatus. According to this arrangement, a
substrate table WT and/or a patterning device table MT are/is
arranged to be moved through a setpoint profile. A displacement
device 10a or 10b is provided for moving the substrate table WT or
patterning device table MT respectively under the control of a
control signal provided by a feedforward control system 12. The
setpoint profile is input via device 18, which may be an input
device such as an external computer, or a storage device. On the
basis of: i) the setpoint profile, which may consist of a plurality
of target states (positions, velocities and/or accelerations, for
example) for the substrate table WT and/or patterning device table
MT to be moved through in a corresponding sequence of target times;
ii) feedforward data provided by a storage device 16; and iii)
substrate table and/or patterning device table state data (e.g.
position, velocity and/or acceleration) provided by a measuring
device 14, the control signal provided to the displacement device
from the feedforward control system is adapted to achieve the
desired movement. As mentioned above, the feedforward data used by
the feedforward control system 12 may include two components: a
component based on system specific knowledge, which in general is
independent of the particular setpoint profile, and a component
that is dependent on the setpoint profile, typically derived from
prior measurements, such as iteratively learned data.
[0044] According to this embodiment, the feedforward control system
12 includes a signal generating part (or signal generator) that is
configured to identify time segments of the setpoint profile that
correspond to entries in a library of feedforward data stored in
the storage device 16. When such a time segment is detected, the
correspond entry in the library of feedforward data is extracted
and used by the feedforward control system to derive a control
signal for the displacement device 10a/10b that corresponds to the
same segment of the setpoint profile. For example, referring to the
setpoint profile illustrated in FIG. 2, the storage device 16 may
be arranged to contain a library entry of feedforward data for the
time segment 2 and for the time segment 6, representing the
constant acceleration phase and the constant deceleration phase.
During these particular time segments, the feedforward control
system will operate based on the corresponding feedforward data
segments extracted from the library. The same two library segments
may be used effectively as feedforward data for setpoint profiles
having a longer or shorter constant velocity segment 4, without
having to derive new feedforward data.
[0045] FIG. 4 illustrates in more detail how the feedforward
control system 12 may be configured to operate. According to this
embodiment, a signal generating part (or signal generator) 20 is
provided that receives input 38 including the setpoint profile from
the setpoint profile device 18 and is connected to the storage
device 16 containing the library of feedforward data. The signal
generating part (or signal generator) 20 includes a trigger
detection system 22, which detects segments of the setpoint profile
that correspond to entries in the library of feedforward data. This
operation may be carried out based on recognition of characteristic
shapes of setpoint profile segments, for example, or may operate by
recognizing triggers that have been deliberated inserted in the
setpoint profile, for example identifying the start of a segment of
the setpoint profile for which it is intended to store a
corresponding entry in the library of feedforward data. For
example, two feedforward tables, I and II, may be used that are
triggered in the sequence I, -I, II, -II by acceleration trigger
points in the setpoint profile. More generally, where the setpoint
profile is defined relative to a time axis, any point on the time
axis may be made available as a potential trigger. An input device
24 is provided that is configured to insert entries from the
library into a feedforward signal 30 when a corresponding segment
is detected by the trigger detection system 22. Where a given
setpoint profile can be built up entirely from elements contained
in the library of feedforward data, the feedforward signal 30 may
be input substantially continuously during processing of the
setpoint profile by the feedforward control system 12. Otherwise,
the feedforward signal 30 will be supplied intermittently.
[0046] The remaining aspects of the control scheme are as follows.
At point 32, a setpoint profile signal from the setpoint profile
device 18 is compared with a measured position signal of the
component to be moved (in the case of the embodiment of FIG. 3,
this signal will be provided by one or both of the measuring
devices 14) and an error signal 34 is forwarded to controller 28.
This feedback control is desirable to account for non-repetitive
disturbances that are always present to some extent. The controller
28 also accounts for any mismatch in the applied setpoint
feedforward (if the feedforward signal 30 is perfectly derived then
no mismatch will occur--see below). The output from controller 28
is added to a feedforward signal 36 representing, in the present
embodiment, an inertial feedforward based on known physical
properties of the system in question, such as the mass of the
substrate table and associated components, and to a feedforward
signal 30 from the signal generating part (or signal generator) 20.
The resultant signal is passed to the displacement device 10a/10b
which effects a change in the position x, velocity v and/or
acceleration a of the component to be moved.
[0047] According to an embodiment of the invention, entries in the
library of feedforward data stored in the storage device 16 may be
derived on the basis of an iterative learning control algorithm
based on the error signal 34.
[0048] The algorithm is shown in block diagram representation in
FIG. 5,where, e.sub.y(k) represents an n-sample error colon (an
array of data points, which have been sampled at a specific
sampling frequency, for example 5 kHz) for the k-th iteration with
e.sub.y(0)=e.sub.yo, Filc(k) represents an n-sample colon of
learned control forces with Filc(0)=0, L represents a learning gain
matrix (which may be a nonlinear function of the error e.sub.y(k)),
z.sup.-1 a one-sample time delay in a z-transform notation, I a
unitary matrix, and Sp a so-called Toeplitz matrix representing the
closed loop process sensitivity dynamics. Basically, Sp describes
the effect of a force on the closed-loop error, L the learning
gain, i.e. the gain used to reduce this error, and z.sup.-1I is
part of an update mechanism needed to update the feedforward signal
to be constructed. The algorithm now works as follows. Starting
with an array of collected errors during a learning profile, in the
first run an array of feedforward forces is computed by multiplying
the error array with the learning gain matrix L. In the second run,
the same learning profile is applied but now with the first run
feedforward forces applied to the system in a synchronized manner.
This will, generally, yield smaller errors than before but due to
the finite gain constraints of L the errors will not equal zero.
Therefore the resulting errors will again be multiplied with L
giving an additional array of feedforward forces which will be
added to the existing array of feedforward forces. The adapted
array of feedforward forces is applied to the system in a third run
(again under the same learning profile) and so on. This procedure
is repeated run after run under sufficient convergence of either
the resulting array of errors or the array of applied feedforward
forces
[0049] According to this embodiment, a reference setpoint profile
is chosen which can be decomposed into parts that form the building
blocks for other setpoint profiles that need to be handled by the
learning design. In the example of FIG. 6, such a reference profile
is represented by a forward and backward motion between about -150
mm and about +150 mm. In the Figure, curve 40 represents the
position as a function of time. Acceleration is applied as a pulse
or hub 42, including a region of constant rate of increase of
acceleration 44, a plateau of constant acceleration 46, and a
region of constant rate of decrease of acceleration 48. A
corresponding negative acceleration pulse 50 causes the velocity to
return again to zero at the end of the stroke. For this profile,
basically two error regions are remarkable, namely the region 52
during constant positive velocity and the region during constant
negative velocity (not shown); in these regions, it is particularly
desirable that the errors be kept small. This is particularly the
case in lithographic applications, because the constant velocity
region is frequently where the optimal quality exposure can be
carried out. For the present example, the reference profile is
decomposed into these two parts. Each of these parts is subjected
to the learning algorithm in order to derive learned feedforward
data. Both learned feedforward data segments are stored in separate
tables and may be synchronised before subsequently being applied to
the system. The feedforward data is synchronized with the
acceleration (setpoint) profile exactly in the same way as it is
obtained during learning; in general, the corrective forces
represented by the generated feedforward data should be matched
with the errors that they should compensate for.
[0050] FIG. 7 shows the result of using two learned feedforward
data segments: one for the stroke characterized by positive
displacement (i.e. from the start of the first positive
acceleration hub 60 to the end of the first negative acceleration
hub 62) and one for the stroke characterized by negative
displacement (i.e. from the start of the second negative
acceleration hub 64 to the end of the second positive acceleration
hub 66). In the uppermost figure, the learned feedforward data
segments are applied on the reference setpoint profile (from about
-150 mm to about 150 mm and back to about -150 mm) that was
actually used for the learning. It can be seen that up to the first
negative acceleration hub 62, the error (continuous line 70) is
practically zero as compared to the error before learning (broken
line 72). At the start of the second negative acceleration hub 64,
i.e. at the transition between the two different feedforward data
segments, a small transition phenomenon is recognised. Beyond this
phenomenon, the error remains practically zero until the point
where the learned force is no longer applied, i.e. at the start of
the last acceleration hub 66.
[0051] In the middle graph of FIG. 7, the two learned feedforward
data segments are partly applied on the first trial profile, i.e. a
shorter profile between about -125 mm and about 125 mm. It can be
seen by the difference between the error using the learned data
derived from the reference setpoint profile (continuous line 70)
and the error before learning (broken line 72) that a significant
improvement can be obtained in settling times. That is, the large
peaks in the error signal induced by the non-smooth transitions in
the acceleration profile are removed after applying a learned
signal constructed from two different feedforward data segments
corresponding to a different profile. Note that the transition
phenomenon can no longer be recognized for the noise in the error
signal. In fact, by choosing the transition between the different
learning tables at the rather arbitrary point of a deceleration
hub, the transition phenomenon, in principle, occurs in that part
of the acceleration profile that is not directly related to
achieving performance in lithography systems (this is because it is
generally the case that imaging is carried out during the regions
of constant velocity on the positive or negative stroke; where this
is the case, it does not matter if the error deteriorates during
phases of acceleration, deceleration or changes of direction as
long as the error during the constant velocity phase is improved).
For this reason, it may be desirable to avoid locations for the
transition near the start of the region of constant velocity. In
the lowermost graph of FIG. 7, the ability to deal with setpoint
variations using a library of multiple feedforward data segments is
shown for a second trial profile between about -100 mm and about
100 mm. It is concluded that setpoint variations can be handled
effectively under the assumption that the reference profiles share
common features which can be learned separately.
[0052] In another embodiment, instead of the entire acceleration
phase being used and subsequently stored for learning, merely the
jerk/djerk phase and part of the corresponding constant velocity
regions are used. For the jerk phase, i.e. the first entry in the
library of learned feedforward data, this is shown in FIG. 8. The
uppermost graph represents a plot of error against time: the
thinner line 80 represents the error before learning and the
thicker line 82 represents the (improved) error achieved using
learned feedforward data. The lowermost graph shows the variation
of learned feedforward data (also referred to as a corrective
"force") with time. After the transition point 84, no learned
feedforward data is available and it can be seen that the error
curves 80 and 82 in the uppermost graph lie on top of each other
thereafter. This embodiment is capable of dealing with velocity
variations since merely the jerk/djerk phases are considered and
not the constant acceleration phase in between (which may therefore
be made longer or shorter while still being amenable to feedforward
data segments based on the jerk/djerk phases).
[0053] Embodiments of the present invention may be applied in the
field of lithographic motion systems like the control of reticle
stages or substrate tables, as mentioned above. The system may also
be used in stages for electron microscope imaging, MagLev stages
for laser cutting, or repetitive motion systems in a more general
perspective. Other fields of application include, for example,
UHP-lamp control where an iterative learning control scheme as
previously been introduced.
[0054] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin-film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "wafer" or "die" herein may be considered as
synonymous with the more general terms "substrate" or "target
portion", respectively. The substrate referred to herein may be
processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may also
refer to a substrate that already contains multiple processed
layers.
[0055] Although specific reference may have been made above to the
use of embodiments of the invention in the context of optical
lithography, it will be appreciated that the invention may be used
in other applications, for example imprint lithography, and where
the context allows, is not limited to optical lithography. In
imprint lithography a topography in a patterning device defines the
pattern created on a substrate. The topography of the patterning
device may be pressed into a layer of resist supplied to the
substrate whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
[0056] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 355, 248, 193,
157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g.
having a wavelength in the range of 5-20 nm), as well as particle
beams, such as ion beams or electron beams.
[0057] The term "lens", where the context allows, may refer to any
one or combination of various types of optical components,
including refractive, reflective, magnetic, electromagnetic and
electrostatic optical components.
[0058] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein.
[0059] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
* * * * *